1. Field of the Invention
The present invention relates to a color conversion film which can be fabricated by a coating process and is formed using a polymeric dye material having a high color conversion efficiency. The invention also relates to a multicolor light-emitting organic EL device which is formed using such a color conversion film.
2. Background of the Related Art
Research targeted at the practical application of organic EL elements has been actively carried out in recent years. Because organic EL elements are able to attain a high current density at a low voltage, there is an expectation that such elements will achieve high luminance and luminescence efficiency. The practical use of organic multicolor EL displays capable of high-definition multicolor or full-color display is especially anticipated. One example of a process for achieving an organic EL display capable of multicolor or full-color display is a process that uses a plurality of color filters which allow light of specific wavelengths to pass through (the color filter process). When the color filter process is employed, it is desired that the organic EL element used be one which emits so-called white light; that is, which emits multicolor light that includes a good balance of the three primary colors of light (red (R), green (G) and blue (B)).
Methods that have been investigated for obtaining multicolor light-emitting organic EL elements include a method in which an emissive layer containing a plurality of light-emitting dyes is used and the plurality of light-emitting dyes are subjected to excitation at the same time (see Patent Documents 1 and 2). In another method which uses an emissive layer containing a host light-emitting material and a guest light-emitting material, the host light-emitting material is excited and induced to emit light while at the same time energy transfer is carried out to the guest material, causing it to emit light (see Patent Document 3).
However, the above-described multicolor light-emitting organic EL elements either rely on the simultaneous excitation of a plurality of light-emitting materials or rely on energy transfer between a plurality of light-emitting materials. It has been reported that in such devices the balance of emission intensities between the light-emitting materials changes as the drive time elapses or with changes in the energizing current, which may alter the hue that is obtained.
Another approach that has been proposed for obtaining multicolor light-emitting organic EL elements is a color conversion method which uses both a monochromatic light-emitting organic EL element and a color conversion film (see Patent Documents 4 to 6). The color conversion film used is a layer containing one or a plurality of color conversion substances which absorb short-wavelength light and convert it to long-wavelength light.
Patent Document 1: Japanese Patent No. 2991450
Patent Document 2: Japanese Patent Application Laid-open No. 2000-243563
Patent Document 3: U.S. Pat. No. 5,683,823
Patent Document 4: Japanese Patent Application Laid-open No. 2002-75643
Patent Document 5: Japanese Patent Application Laid-open No. 2003-217859
Patent Document 6: Japanese Patent Application Laid-open No. 2000-230172
However, when a color conversion film is formed using a single color conversion material, molecules of the color conversion material mutually aggregate; as the energy of the absorbed light is repeatedly transferred between the same molecules, a phenomenon known as concentration quenching arises in which the color conversion material deactivates without light emission. To suppress this phenomenon, the color conversion material is dissolved or dispersed in a medium to lower the concentration and thereby suppress contact between molecules of the color conversion material (see Patent Document 5).
Here, lowering the concentration of the color conversion material reduces the absorbance of the light to be absorbed, as a result of which a sufficient conversion light intensity is not obtained. To address this issue, the color conversion film is made thicker, increasing the absorbance and thereby maintaining the efficiency of color conversion. A number of problems are associated with the use of such a thick color conversion film (film thickness, about 10 μm), including the occurrence of electrode pattern disconnects in stepped areas, the difficulty of achieving a high definition, and moisture or solvent residues in the film (when the film is combined with an organic EL device, residual moisture or solvent alters the organic EL layer, resulting in display defects). At the same time, from the standpoint of reducing the viewing angle dependence, there exists a competing desire to make the color conversion film thinner.
It is therefore an object of the present invention to provide a color conversion film which is capable of maintaining a sufficient conversion light intensity without the thickness of the film being increased. A further object of the invention is to provide a color conversion film which can be fabricated by a low-cost wet process. A still further object of the invention is to provide a multicolor light-emitting organic EL device which uses such a color conversion film; that is, to provide a multicolor light-emitting organic EL device which has an excellent viewing angle dependence and, because the hue does not change as the drive time elapses or with changes in the energizing current, exhibits stable emission characteristics over an extended period of time.